The present invention relates to a misfire detecting apparatus for a multicylinder internal combustion engine. The apparatus serves to detect misfire occurring in the internal combustion engine.
A conventional apparatus for detecting misfire occurring in an internal combustion engine is disclosed e.g. in JP-A-61-258,955. This apparatus detects misfire as follows. When misfire occurs, complete combustion cannot be attained in a combustion chamber of an internal combustion engine, so that the rotational speed of the internal engine (hereinafter refereed to as an engine speed) is lowered. Therefore, instantaneous engine speeds at least at two points in a single ignition or firing cycle are detected to obtain a change in the engine speed. A variation of the engine speed change is compared with a misfire detection value set in view of the operating state of the internal combustion engine. When the former becomes larger than the latter, occurrence of misfire is decided. However, the above conventional apparatus has a following problems. When a vehicle runs on a rough road, the contacting state of wheels on a road surface becomes unstable because of unevenness of the road surface. Namely, the vehicle may leave the road surface or contact strongly with the road surface. Thus, the state of a load on the internal combustion engine changes, so that the rotational speed of a crank shaft becomes unstable. As a result, even when misfire does not occur, a variation of the engine speed change may increase. For this reason, even when misfire does not occur actually, the above conventional misfire detecting apparatus may detect erroneously occurrence of misfire, because of unevenness of the road surface.
Further, when the vehicle is decelerated abruptly, the engine speed lowers abruptly. Thus, even when misfire does not occur, a variation of the engine speed change increases, so that occurrence of misfire may be decided erroneously.